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  <script>
    console.log(typeof NaN); //Number
    //不是一个确切的值
    let a = 'abc'
    console.log(Number(a));
    //NaN,表达式中有不可转化的变量，返回了无效结果
    //NaN不等于自己，不是一个确切的值，代表一个范围
  </script>
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